Or by passing in a dictionary of functions when Validator
is instantiated.

Your functions can use keyword arguments,
but the first argument should always be 'value'.

If the function doesn't take additional arguments,
the parentheses are optional in the check.
It can be written with either of :

keyword = function_name
keyword = function_name()

The first program to utilise Validator() was Michael Foord's
ConfigObj, an alternative to ConfigParser which supports lists and
can validate a config file using a config schema.
For more details on using Validator with ConfigObj see:
http://www.voidspace.org.uk/python/configobj.html